Peace talks conducted with an impostor who posed as a Taliban leader, and which led to a meeting with Hamid Karzai in Kabul and thousands of dollars in "goodwill payments", were started by the Afghan government and approved by the former American commander, Stanley McChrystal, the Guardian has learned.

This account sharply contradicts claims made by the Afghan presidency, which has put the entire blame on Britain, apparently supported privately by US officials.

In fact, the overriding desire to find a negotiated end to the conflict, particularly on the part of David Cameron, appears to have generated credulity on all sides, and led to an embarrassing debacle that has lessened trust and set back hopes of meaningful negotiations in the near future.

Sources close to the contacts said the impostor, who claimed to be Mullah Akhtar Mohammad Mansour, the Taliban's deputy leader, was originally introduced by an insurgent commander in Kandahar to the then Afghan interior minister, Hanif Atmar.

This Taliban commander, Muhammad Aminullah, is close to the movement's overall leader, Mullah Omar, and has led some of the fiercest Taliban fighting in the Zhari and Panjwai districts of Kandahar province. When he was picked up and held in a Nato raid in January this year, the Afghan government complained that he was a longstanding channel of Atmar's to the Taliban, and asked for him to be freed. In return, Aminullah offered contacts with Mansour, suggesting he might be open to political talks. The deal was approved by McChrystal, then the commander of Nato and US forces in Afghanistan, and a supporter of reconciliation efforts.

McChrystal asked MI6 to develop the contacts, rather than go to the CIA, which was not empowered by the necessary White House directive to enter into direct talks with Taliban officials. The absence of such a "presidential finding" is seen by many diplomats on both sides of the Atlantic as an obstacle to progress towards a political settlement.

At this point, MI6, delighted to have been given the mission, appears to have got carried away with enthusiasm for the "breakthrough", and brushed aside doubts raised by both US and British officials about "Mansour's" credibility. "Our friends got very excited," one official involved in the discussions recalled. "I remember everyone being very pompous and secretive about this." McChrystal's successor, Gen David Petraeus, is believed to have had doubts about Mansour's identity, but ultimately encouraged the contacts and discreetly publicised them.

Last night the head of the US military, Admiral Mike Mullen said the US had suspected the self-described Taliban leader was an imposter. "There were very early initial suspicions. And it took a little while to verify who he was or who he wasn't. And, in fact, it turns out he wasn't the guy he was claiming he should be."

After the coalition took office in May, both Cameron and William Hague were briefed about the talks with Mansour. The prime minister's eagerness to pursue a negotiated settlement contributed to an echo chamber in which more cautious voices were drowned out.

A series of meetings at a Nato military base in Kandahar culminated in the supposed Taliban leader being flown to Kabul in a British military plane to meet Karzai just over three months ago.

In that meeting, and at some of the preliminary meetings, the impostor (reported by the Washington Post, citing Afghan intelligence, to be a grocer from Quetta), was given tens of thousands of dollars as a reward for attending and as encouragement to develop the dialogue.

It is unclear how much of that money was paid by Britain and how much by Karzai, who keeps his own fund, partially financed by Iran, for such purposes. The US has insisted no American money was used.

It was at the meeting with Karzai that "Mansour's" identity was definitively challenged, leading to his unmasking earlier this week.

McChrystal, who has retired from the US army, could not be reached for comment and Atmar, who was in London this week, did not reply to emails seeking comment.

Interviewed in today's Washington Post, Karzai's chief of staff, Mohammad Umer Daudzai, squarely blamed the British for the fiasco. "This shows that this process should be Afghan-led and fully Afghanised," Daudzai said. "The last lesson we draw from this: international partners should not get excited so quickly with those kind of things … Afghans know this business, how to handle it."

Another Afghan official echoed that account, telling the Guardian: "Generally speaking, British intelligence has been the main director and architect of the peace plan; and in this particular case the mediators were British."

The official also blamed the Pakistani intelligence agency, the ISI, which he said introduced the fraud to MI6. The Guardian, however, could find no confirmation of any role played by the ISI, which is frequently blamed for setbacks by the Kabul government.

British intelligence is conducting an inquiry into the episode, in part to uncover the motive. One theory is that it was an exercise in kite-flying by the Taliban, to discover what Kabul and the British were offering, without risking a senior figure in the movement. Taliban leaders have been wary about attending meetings with would-be mediators, fearing they are on a Nato hit-list, known as the Joint Priority Effects List. A Nato source said: "If you look at it from their point of view, as soon as they turn up for a meeting, they give us an eight-digit map reference of where they are. This, on the other hand, is no risk."